Patent GB-800304-A (GB)
Title: Improvements in or relating to apparatus and methods for testing optical systems, lenses and the like
Inventor: HOPKINS HAROLD HORACE
IPC: G01B9/02
Filed: 1955-04-04
Abstract: 800,304. Photo-electric testing. NATIONAL RESEARCH DEVELOPMENT CORPORATION. July 3, 1956 [April 4, 1955], No. 9837/55. Class 40 (3). [Also in Group XX] Apparatus for testing an optical system comprises a straight line or point source of light, means for dividing the amplitude of the wavefront of the beam passed through or reflected by the system to form two component beams having similar wavefronts, means for directing the component beams to overlap with their wavefronts parallel but in sheared relation so as to interfere, means for varying the amount of shear between the wavefronts or (or and) means for varying the optical path lengths of the component beams before they overlap and means for measuring the consequent change in the total light flux in the interferogram produced by the interfering beams or in a selected part of that interferogram. The light measurements may be made photoelectrically. As shown in Fig. 1, light from a lamp 11 passes a slit 14 and is collimated by a lens 15 to be plane polarized by a Nicol prism 16 and divided by a Wollaston prism 17 into two slightly divergent beams. One beam 22 is reflected by mirrors 19 and 24 to be focused by a lens 26 at a slit 28 at the focus of the lens 34 under test. The beam 36 from the lens is divided by a semi-transparent reflector 38 into two component beams falling respectively on right-angle mirror systems 43, 44 which reflect the beams back to the reflector 38 in sheared relation. This is illustrated in Fig. 2 which shows the wavefront 30 of the beam 36 from the lens 34 divided into beams 41, 42 by the reflector 38 and returned as beams 41a, 42a giving wavefronts 40 and 50 sheared laterally, one with respect to the other, thereby producing an interferogram focused by lens prism 52 on to diaphragm 54, Fig. 1. Light from this interferogram is focused by a lens 55 on to a transparent rod 57 ending in a sphere 58 light from which passes to a photo-cell 61 which gives a measure of the total light flux through the diaphragm 54. Alternatively the scalloped grating described in Specification 800,303 may be used. The variations of light flux produced as the wavefront shear is varied by moving the reflectors 43, 44 in the direction of the arrows in Fig 1, and as the path difference is varied by movement of the reflectors perpendicular to the arrows, give information from which the aberrations of the lens 34 may be deduced. The light flux from the interferogram may be compared with that from a similar interferogram produced by a standard lens 69 by light from the beam 21 produced by prism 17 and reflected by mirrors 18, 23 and 72 on to the reflector 38, a plane polarizer 33 being rotated in the light from the prism 17 to cut off the beams 21 and 22 alternately, and the Nicol prism 16 may be rotated to vary the relative intersites of the beams 21, 22 to provide a balance between the light flux from the two interferograms. The reflectors 43, 44 may be moved by means of the screw arrangement shown in Fig. 4A.